1. Field of the Invention
This invention relates to magnetic transducers particularly applicable to magnetic disk drive systems. More particularly, the invention relates to devices and methods of manufacturing thin film inductive heads having a narrow pole tip for high density data transfer in a magnetic disk drive system.
2. Description of the Related Art
Thin film magnetic read/write heads are used for reading and writing magnetically coded data stored on a magnetic storage medium such as a magnetic disk or a magnetic tape. There is a continuing strongly-felt need for increasing the data storage density in such media. Most efforts to increase magnetic storage density involve techniques for increasing the areal bit density in the magnetic medium.
In rotating magnetic disk drives, the areal density is the product of the number of flux reversals per millimeter along a data track and the number of tracks available per millimeter of disk radius. Thus, high areal data storage density requires recording heads with high linear resolution and narrow track width.
A thin film recording head includes first and second pole pieces which are magnetically coupled together at a pole tip region and at a back gap. In the pole tip region, the first and second pole pieces provide first and second pole tips which are separated by a thin non-magnetic gap layer. The thickness of the gap layer between the second pole tip and first pole tip and the configuration of the second pole tip are the most crucial elements in thin film write heads. The thickness of the gap layer at the head air bearing surface determines the linear density of the head, namely how many bits per linear unit length along a data track of a magnetic medium the head can write. The width of the second pole tip determines head track width, which establishes how many data tracks across the width of a magnetic medium per unit length can be written by the head. The product of these two factors is areal density. One factor in increasing areal bit density is achieving a narrow track width second pole tip.
In one method of fabrication, a second pole tip is constructed individually and then the remainder of the second pole tip piece is "stitched" to the second pole by ordinary photolithography as described in U.S. Pat. No. 5,282,308 issued Feb. 1, 1994, and assigned to the same assignee as that of the present invention. In another process the second pole tip and the second pole piece are plated simultaneously in the same process step. However, prior art methods of constructing the second pole piece and the second pole tip with the same process steps have not provided a high resolution second pole tip. When the second pole piece and the second pole tip are constructed simultaneously by ordinary photolithography, a photoresist layer is spin-coated over the body portion and the pole tip region of the head to provide a plating mask. The photoresist layer is located above a gap layer in the pole tip region and above a stack of insulation/coil layers in the body, i.e., the coil region of the head. The insulation stack is typically 7-8 microns (.mu.m) thick above the gap layer and has a marked slope as the first insulation layer transitions to its apex or the point where the slope ends. Since photoresist is applied on a wafer by spin-coating, it partially planarizes across the body portion and the pole tip region causing the photoresist over the pole tip region to be considerably thicker than the photoresist over the body portion of the head. The thickness of the photoresist in the body portion of the head is dictated by the desired plated metal thickness of the second pole piece. For example, if the second pole piece in the body portion is to be 4 .mu.m thick, the photoresist layer would have to be approximately 4.5 .mu.m thick to properly encapsulate the entire height of the plated material. With a typical insulation stack height of about 8 .mu.m, this results in a photoresist layer that could be as thick as 12.5 .mu.m in the pole tip region. This photoresist thickness in the pole tip region plus the steep slope of the first insulation layer near the pole tip region makes it very difficult to construct a narrow track width second pole tip with subsequent photolithography steps. After the photoresist layer is deposited, it is patterned by exposure to light in one or more areas which are to be removed by a subsequent step of dissolving the exposed photoresist. Because of the thickness of the photoresist in the pole tip region, the intensity of the light for patterning has to be high in order to penetrate the full depth of the photoresist. When the intensity of the light is high, the narrow slits in the mask employed for patterning miniature features in the photoresist introduce diffraction of the exposure light at the edges of the slits. This results in poor resolution. A more serious problem, however, is the reflection of light into the pole tip region from the sloping insulation layers behind the pole tip region. The reflection causes notching of the photoresist layer resulting in poor definition of the pole tip. Plating after this type of patterning results in a second pole tip with irregularly shaped sidewalls and a poorly defined width.
In a viable manufacturing process for making high resolution thin film heads, the aspect ratio, which is the thickness of the photoresist layer in the pole tip region with respect to the track width (i.e, the width) of the pole tip, should be less than about 4 to 1 using conventional photolithographic processes. Experience indicates that in a data recording head for one gigabit areal density recording, the pole tip width should be 3 .mu.m which limits the photoresist thickness to 12 .mu.m or less. A recording head for two gigabits requires a pole tip width of 1.8 .mu.m with the photoresist thickness less than 7.2 .mu.m. A recording head for 5 gigabits has been found to have a pole tip width of 0.9 .mu.m with a photoresist less than 3.6 .mu.m.
It is apparent that a maximum aspect ratio of 4 to 1 cannot be maintained for high density recording heads in the gigabit range using conventional photolithographic processes when a photoresist layer planarizes to a greater thickness over the second pole tip than over the body region.
Accordingly, a solution for reducing the aspect ratio in a process which constructs the second pole tip at the same time the second pole piece in the body region is constructed is needed which will make available high density recording heads in the gigabit range using conventional photolithic processes.